1. Field of the Invention
The present invention relates to a driving force transmission device for four-wheel-drive vehicle based on the two-wheel drive of front wheels enabling to change a two-wheel drive and a four-wheel drive, more specifically, to a driving force transmission device for four-wheel-drive vehicle in which the rotation of the parts not operative for the transmission of the driving force in the two-wheel drive is stopped.
2. Description of the Related Arts
As one of driving force transmission devices for four-wheel-drive vehicle in which front wheels are driven in the two-wheel drive, and the distribution control of the driving force to the rear wheels is performed in the four-wheel drive by a multi-disc clutch mechanism, in the four-wheel-drive vehicle based on the two-wheel drive of conventional so-called on-demand type full-time front wheels, for example, the device shown in FIG. 1 is known.
FIG. 1 shows a driving force transmission device for four-wheel-drive vehicle based on the two-wheel drive of conventional so-called on-demand type full-time front wheels. In FIG. 3, a driving force transmission device 300 is provided in a four-wheel-drive vehicle 302, and the driving force from an engine 304 is changed by a gearbox 306 and input into a front wheel differential 308 and a driving force direction converting unit 310 in the driving force transmission device 300, and the output from the driving force direction converting unit 310 is transmitted to a multi-disc clutch mechanism 314 through a propeller shaft 312. When the multi-disc clutch mechanism 314 is released (disconnected) in the two-wheel drive, the driving force is not distributed to a rear wheel differential 324 but transmitted to the front wheel differential 308, and the front wheel differential 308 absorbs the rotation speed difference between a left front wheel 316 and a right front wheel 318, gives a same torque to the left front wheel 316 and the right front wheel 318, and makes the front wheels rotate. When the multi-disc clutch mechanism 314 is fastened (connected) in the four-wheel drive, the driving force is transmitted also to the rear wheel differential 324 through a drive pinion 320 connected to the multi-disc clutch mechanism 314 and a ring gear 322, and the rear wheel differential 324 absorbs the rotation speed difference between a left rear wheel 326 and a right rear wheel 328, gives a same torque to the left rear wheel 326 and the right rear wheel 328, and makes the rear wheels rotate. In general, in the on-demand type full-time four-wheel-drive vehicle, as the drive modes for the drivers to optionally select by switch operations, a two-wheel-drive mode, a four-wheel-drive auto mode, and a four-wheel-drive lock mode are prepared. The two-wheel-drive mode is the mode that is used in the two-wheel-drive state in which the multi-disc clutch mechanism 314 of the driving force transmission device 300 is released, and selected when to show the maximum running performance on dry pavement roads and the like on which the driving force by four wheels is not necessary for the best mileage. The four-wheel-drive auto mode is the mode in which various running vehicle states are detected by sensors, and the driving force distribution of the multi-disc clutch mechanism 314 to the front and rear wheels are automatically controlled at the most suitable state by an ECU (Electronic Control Unit) based on the detection signals of the sensor, and can be always selected regardless of road surface states. In this mode, the fastening power of the multi-disc clutch mechanism 314 is increased and/or decreased continually by an actuator, and the driving force distribution to the front and rear wheels is controlled between the two-wheel-drive state in which the driving force to the rear wheels is approximately zero and the maximum fastening power. The four-wheel-drive lock mode is the mode in which the fastening power of the multi-disc clutch mechanism 314 is retained at its maximum regardless of the vehicle states detected by the various sensors, and this mode is selected when to show running characteristics as the four-wheel drive on rough roads and the like at the maximum. However, in such a conventional driving force transmission device for four-wheel-drive vehicle based on the two-wheel drive of front wheels, even in the two-wheel-drive mode, since the driving force from the gearbox 306 makes a driving force direction converting unit 310, a propeller shaft 312 and the driving side (front wheel side) of the multi-disc clutch mechanism 314 rotates, and the left rear wheel 326 and the right rear wheel 328 and the rear wheel differential 324 are directly connected, the left rear wheel 326 and the right rear wheel 328 rotate, and consequently, the rear wheel differential 324, the drive pinion 320 and the driven side (rear wheel side) of the multi-disc clutch mechanism 314 also rotate. That is, even in the two-wheel drive in which the multi-disc clutch mechanism 314 is released and the driving force is not transmitted to the rear wheels, and even if the multi-disc clutch mechanism 314 is released completely, the respective structural elements in the rear wheel driving force transmission section 330 including the driving force direction converting unit 310, the propeller shaft 312, the multi-disc clutch mechanism 314, the drive pinion 320 and the rear wheel differential 324 rotate, and this leads to a mileage fall by stirring resistance of the oil in this section or friction loss of bearings, which has been still another problem in the prior art. Further, the multi-disc clutch mechanism 314 includes a plurality of clutch discs, and although these clutch discs are lubricated and cooled by oil, since the so-called dragging torque that occurs by the viscosity resistance of the oil arising from the rotation speed difference between the driving side and the driven side of the clutch discs and the friction loss by the contact between the clutch discs is larger than the friction torque of the drive pinion 320 and the rear wheel differential 324, even if the left rear wheel 326 and the right rear wheel 328 and the rear wheel differential 324 are disconnected, the drive pinion 320 and the rear wheel differential 324 are rotated by the multi-disc clutch mechanism 314, and this deteriorates the mileage, which has been yet another problem in the prior art. In order to decrease this dragging torque, if the supply of the oil to the multi-disc clutch mechanism 314 is stopped, or if the multi-disc clutch mechanism is used with an extremely reduced quantity of the oil, it is possible to decrease or eliminate the dragging torque that occurs by the viscosity resistance of the oil. However, there is a fear that the multi-disc clutch mechanism 314 may be seized if sufficient lubrication is not performed at the time of the driving force distribution control. Furthermore, as a method to ensure sufficient intervals among the respective clutch discs in the released state of the multi-disc clutch mechanism 314, a method is suggested in which a ring spring 332 formed in a wave shape in the circumferential direction as shown in FIG. 2 is put between adjacent clutch discs of the driving side or adjacent clutch discs of the driven side, however, according to this method, it is possible to ensure the sufficient intervals between the clutch discs of the driving side or the clutch discs of the driven side, but it is not possible to prevent the clutch discs of the driving side from contacting the clutch discs of the driven side. In the four-wheel-drive auto mode, the driving force is transmitted to the rear wheels 326 and 328, and even if the transmitted driving force is lowest, a loss larger than that at the time of the two-wheel-drive mode occurs in the rear wheel driving force transmission section 330, and consequently, the mileage becomes worse than that at the time of the two-wheel-drive mode. In this mode, the driving force distribution is automatically controlled at the most suitable state even if the driver does not perform any operation, however, in the driving condition that does not need the four-wheel drive such as running on dry pavement roads, when the driver forgets the switch change to the two-wheel-drive mode, the running is continues in a poor mileage state, which has been yet another problem in the prior art. In this case, by switching over the two-wheel-drive mode and the four-wheel-drive auto mode automatically depending on driving conditions, it is possible to improve the mileage regardless of the switch operation by the driver, but for that purpose, it is necessary to quickly change the two-wheel drive and the four-wheel drive. As another method to decrease the dragging torque of the multi-disc clutch mechanism 314, there may be a method in which the distance of the approach section shifting from the standby state at the time of the two-wheel drive to the fastening state at the time of the four-wheel drive of the multi-disc clutch mechanism 314, that is, the so-called end play, is made large, and the intervals among the respective clutch discs are ensured sufficiently in the state in which the multi-disc clutch mechanism 314 is released at the time of the two-wheel drive, thereby the clutch discs are prevented from contacting one another. However, when the end play is made large, it takes much time in changing the mode from the two-wheel drive to the four-wheel drive, which decreases the running performance.